Isometric Drawing-Aid Functions CADD provides a number of special functions that simplify isometric drawing. The following functions are available in most CADD programs:

• Isometric grid• Fixed cursor direction• Isometric circles, text and dimensions• 2D drawings to isometrics conversion Isometric Grid CADD allows you to draw a grid tilted 30 in both directions. The intersections of the grid can be used to draw isometric objects. You can draw a top isoplane, right isoplane or a left isoplane using these grid points. Fig. 7.4 shows an example of isometric shapes drawn using the grid points. The grid can be drawn at any specified distance as needed. When you are finished drawing an isometric, you can erase the grid.

Fixed Cursor Direction:

CADD allows you to fix the direction of the cursor at specific angles. This technique is commonly called setting the orthogonal (ortho) or constraints in many CADD programs. When the direction of the cursor is fixed, i.e., the ortho is turned on, it can move only in preset directions. This makes it easier to draw straight lines, just like using a T-square and a triangle. When working with isometrics, you can fix the direction of the cursor to multiples of 30 angles, e.g. 30, 60, 90. This enables you to easily draw top isoplanes, left isoplanes and right isoplanes (See Fig. 7.5).

Isometric Circles, Text and Dimensions:

A circle appears as an ellipse when drawn in different isoplanes (See Fig. 7.6). Similarly, text and dimensions can be skewed to match the tilt of an isoplane. CADD’s special isometric functions allow you to draw isometric circles, text, dimensions, symbols, etc.

To draw an isometric circle, you need to specify the radius of the circle and the isoplane to which it is to be drawn. CADD automatically draws an ellipse based on the orientation of an isoplane. Similarly, you can draw text and dimensions to match the tilt of an isoplane. This makes the text and dimensions appear to be lying parallel to a specified isoplane. Some examples of isometric text are shown in Fig. 7.6.

2D Drawings to Isometric Conversion:

Special add-on programs are available that enable you to convert 2D plans and elevations into isoplanes. You can combine different isoplanes to develop an isometric. For example, you can convert a plan diagram into top isoplane and the side elevations to left and right isoplanes. CADD automatically twists all the objects of the drawing to match the tilt of the isoplane. You can place the right and left isoplanes on appropriate sides of the top isoplane. This creates a skeleton of the isometric in a few simple steps. You can finish the isometric by drawing the rest of the objects of the drawing.

Fig. 7.7 illustrates how you can draw an isometric of a building block from an existing 2D plan and elevations. Take the plan diagram and convert it into a top isoplane. This will serve as the base of the building. Convert the elevations - Side 2, Side 5 and Side 7 to right isoplanes. Move the isoplanes of the elevations on the appropriate sides of the top isoplane. This completes the basic structure of the isometric. Use this basic structure to complete the rest of the isometric.

3D Modeling:

CADD’s 3D modeling capabilities allow you to create 3D images that are as realistic as the actual objects. These images are called 3D models because, just like a physical model, they can be rotated on the screen. You can display views from a 3D model, such as isometrics or perspectives, from any angle with a few simple steps.

3D modeling is usually a separate CADD module that has its own set of functions. Some manufacturers market 2D programs and 3D programs as separate packages while others combine them into a single program.

The 3D models fall into the following categories:

• Wire-frame models• Surface models• Solid models

When you draw a model with lines and arcs, they are called wire-frame models. These models appear to be made of wires and everything in the background is visible. This does not create a very realistic effect.

Surface models are more realistic than wire-frame models. They are created by joining 3D surfaces rather than bare lines and arcs. A 3D surface is like a piece of paper that can have any dimension and can be placed at any angle to define a shape. Just like a paper model, you join surfaces to form a surface model. The views displayed from these models are quite realistic, because everything in the background can be hidden.

Solid models are considered solid inside and not hollow like a surface model. They appear to be the same as a surface model but have additional properties, such as weight, density and center of gravity, just like that of a physical object. These models are commonly used as prototypes to study engineering designs.

Example: You can draw a 3D model as a wire-frame, a surface model or a solid. To draw a 3D model of a cube as a wire-frame, you need to draw twelve lines by specifying 3D coordinates for each of its points. To draw it as a surface model, you need to draw all six surfaces of the cube. Although you see only three planes of the cube in front, it is essential to draw all the planes when drawing a 3D model. This ensures that a realistic view is displayed when it is rotated to display a view from any angle. When drawing a solid you can also specify its material.

Important Tip:

For general 3D drawings, wire frames and surface models are used. You start with a wire-frame model and then fill in spaces with 3D surfaces to make it more realistic. Working with 3D Coordinates

3D coordinates are measured with the help of three axes: X, Y, and Z. The axes meet at a point in the shape of a tripod as shown in Fig. 7.9. This point is called the origin point, which is the 0,0,0 location of all coordinates. All distances can be measured using this point as a reference.

The three axes form three imaginary planes: XY plane, XZ plane and YZ plane. The XY plane is the horizontal plane and the XZ and YZ are the two vertical planes. When you need to draw something horizontal, such as the plan of a building, you draw it in the XY plane using X and Y coordinates. This generates a plan view. When you need to draw something vertical, such as an elevation of a building, you draw it using the XZ or YZ planes.

Example: To draw a line in 3D, enter two end points defined with X, Y and Z coordinates. If you need to draw the line lying flat on the ground (XY plane), the Z coordinate for both the end points of the line is zero. If you want to draw the same line at 10’-0” above the XY plane, enter the Z-coordinate for both the end points as 10’-0”.

Cartesian coordinates are based on a rectangular system of measurement. In Chapter 2 “The CADD Basics”, we discussed how Cartesian coordinates are used in 2D drawings. The same principle is applied to enter 3D coordinates with the exception that you need to enter an additional Z coordinate. Positive Z-coordinate values are used when you need to measure distances above the XY plane; negative values are used for the distances below the XY plane. The coordinates can be measured from the origin point (absolute coordinates) or from the last reference location of the cursor (relative coordinates). Coordinate values are entered separated by commas (X,Y,Z).

Fig. 7.10 illustrates the concept of the Cartesian coordinate system of measurement. Diagram A shows how the X and Y coordinates are measured in the XY plane just like 2D coordinates. Diagram B shows how to indicate height with the help of the Z coordinate. A point is located 6 units in the X direction, 4 units in the Y direction and 5.5 units in the Z direction using Cartesian coordinates.

Spherical Coordinates:

Spherical coordinates are based on the longitude and latitude system of measurement (Diagram A, Fig. 7.11). Consider the origin point of the coordinate system at the center of the earth or a transparent globe. Then consider a horizontal plane (XY plane) passing through the center of the globe. To locate a point in 3D, first locate a point in the XY plane by specifying a radius and an angle (polar coordinates). To specify the height, enter an angle up or down from the XY plane (latitude).

Diagram B in Fig. 7.11 illustrates how to locate a point using spherical coordinates. The point is located in the XY plane by entering 6.0 units radius and 210 as the rotation angle (longitude). The latitude is entered as 60.

Note:

Spherical coordinates are not very efficient for drawing purposes. They are commonly used to view a model from different angles.

Cylindrical Coordinates:

Cylindrical coordinates are commonly used to draw cylindrical shapes. They are based on a cylindrical system of measurement. Consider a cylinder placed vertically and the origin point at the center of the cylinder (Fig. 7.12, Diagram A). Cylindrical coordinates are quite similar to spherical coordinates, the difference being that the Z-coordinate is specified by height and not angle.

To enter a point with the cylindrical coordinates, first you need to locate it in the XY plane just like polar coordinates. Then indicate an exact height at that point.